The present invention relates to microelectronic assemblies incorporating springs and to methods and elements used in such assemblies.
Microelectronic elements such as packaged and unpackaged semiconductor chips, multi-chip modules and the like are normally mounted on circuit panels such as circuit boards by bonding terminals of the microelectronic element to contact pads on a circuit board using a bonding material such as solder. Assemblies of this type can fail due to breakage of the bonding material. For example, in a so-called “flip-chip” mounting, an unpackaged, “bare” semiconductor chip is mounted with the contact-bearing front face of the chip confronting the top surface of the circuit board and with contacts of the chip bonded directly to contact pads on the circuit board by masses of solder referred to as solder balls. In the bonding process, the solder balls are melted to form the bond and then solidified by cooling the assembly. As the assembly cools from the solidification temperature of the solder to room temperature, both the chip and the circuit board tend to contract, but the circuit board typically contracts to a greater extent than the chip, because the circuit board typically has a greater coefficient of thermal expansion than the chip. Differential contraction during cooling places mechanical stresses on the solder balls. In service, as the assembly is operated, the components are repeatedly heated and cooled, imposing additional, repeated mechanical stresses on the solder balls. The mechanical stresses applied during manufacture and service can cause the solder balls to break and lead to failure of the assembly. Similar problems arise in mounting other microelectronic elements.
Moreover, it is highly desirable to test microelectronic elements before mounting them to circuit panels, so as to assure that only good elements are included in the larger assemblies on the panels. To test a microelectronic element, reliable temporary connections must be established between the mounting terminals of the microelectronic element and the contacts of a test fixture. Considerable difficulty can be encountered in establishing reliable connections with all of the numerous terminals on a microelectronic element at the same time. For example, where a microelectronic element includes a rigid, theoretically planar array of terminals intended for solder bonding to a circuit panel, some of the terminals may be slightly out of plane. This makes it difficult to establish simultaneous contact with all of the terminals using a rigid test fixture. In some cases, solder balls are mounted to the terminals of the microelectronic element during manufacture. This further complicates the testing problem, because solder can accumulate on the contacts of the test fixture when numerous microelectronic elements are tested in sequence.
Considerable efforts have been devoted heretofore to alleviating these problems. For example, microelectronic elements can be provided in packages having terminals separate from the contacts of the chip or the microelectronic element itself. Certain packaged microelectronic elements sold under the trademark μBGA® by Tessera, Inc. and its licensees have terminals which are movable relative to the chip or the microelectronic element itself. Such movability can alleviate stresses on the solder balls caused by differential thermal expansion and contraction. Moreover, in some cases, movability of the terminals can facilitate engagement of terminals with a test fixture. Most commonly, the terminals are mounted on a dielectric element as, for example, a polymeric sheet or panel and are connected to the contacts of the microelectronic element itself by leads within the package. In certain preferred embodiments, the packaged microelectronic element itself may be approximately the same size as a comparable unpackaged microelectronic element and, accordingly, may occupy little or no additional space on the circuit panel. The techniques used in μBGA® packaged microelectronic elements have been successfully used and widely adopted in the industry. However, despite these improvements, still further improvements and alternatives would be desirable.
Test fixtures having resilient movable contacts have been employed to test packaged and unpackaged microelectronic elements. While some of these test fixtures permit reliable engagement between the terminals of a microelectronic element and the test fixture, such test fixtures can add to the cost of the testing operation. Moreover, improvements in test fixtures do not alleviate the problem of bond failure or the problem of solder accumulation on the test fixture.
Yet another approach which has been adopted is to alter the bond between the microelectronic element and the circuit panel in ways which make the bond more resistant to applied stresses. For example, as taught in Grabbe, U.S. Pat. No. 4,642,889, solder masses interconnecting an electronic element and a circuit panel may include fine reinforcing wires such as copper wires. Allen et al., U.S. Pat. No. 4,705,205 discloses a similar approach in which the solder element may include a metallic strand or strip which, in certain embodiments, is illustrated as a helical element surrounding the solder mass. While addition of reinforcements may increase the reliability of the solder bond, it does not solve the testing problem. Brofman et al., U.S. Pat. No. 5,968,670 employs solder-coated springs in conjunction with ordinary solder masses. During the bonding operation, while the solder masses connecting the terminals of the microelectronic element and the circuit board are in a molten condition, the solder on the springs melts and allows the springs to expand. The expanding springs force the microelectronic element away from the circuit board, thereby stretching the molten solder masses into elongated columns. The solder masses retain the columnar shape when cooled. The elongated, columnar solder masses are more resistant to stresses applied during service. This approach also does not address the testing problem. Solder-coated springs are also used as internal elements of printed circuit boards, as taught, for example, in Dube et al., U.S. Pat. No. 3,509,270 and Beck, U.S. Pat. No. 3,616,532. These arrangements do not address the problems of mounting a microelectronic element to a circuit panel. Other references which discuss springs or wire-like elements as mounting elements include Kanji et al., U.S. Pat. No. 5,067,007; Calomagno et al., U.S. Pat. No. 4,955,523; and Khandros et al. U.S. Pat. No. 6,442,831.